Abstract Cell division requires a complex network of dozens of pathways whose interactions drive key transitions. One of the most critical is the transition from metaphase to anaphase and mitotic exit. The Anaphase-Promoting Complex/Cyclosome (APC/C), an E3 ubiquitin ligase, comprises the central target node of this critical decision point. APC/C activity is sensitive to chromosome congression to the metaphase plate. At metaphase, the APC/C ubiquitylates target mitotic regulators for destruction in the proteasome and induction of chromatid separation and mitotic exit. While regulatory pathways for the APC/C have been identified, complete understanding of how it is tuned to program the metaphase-anaphase transition after chromosome alignment remains unclear. Recent work has implicated the Spindle and Kinetochore Associated (Ska) protein complex as a key element. The studies proposed will clarify the molecular mechanisms by which Ska collaborates with and controls other mitotic regulators, particularly protein phosphatases, governing the temporal and spatial activation of the APC/C to drive the metaphase-anaphase transition. Metaphase, itself, is generally brief, but the Gorbsky laboratory discovered that delays, even short ones, can cause partial or complete chromatid separation, a phenomenon termed ?cohesion fatigue.? Cohesion fatigue may be remarkably common as a source of both numerical aneuploidy and large chromosome deletions, duplications and translocations, particularly in cells transformed by activated oncogenes and the loss of tumor suppressors. The Gorbsky laboratory is taking a broad approach to study all potential inputs that contribute to cohesion fatigue. The laboratory is also mapping the short term and long term consequences of cohesion fatigue to determine how it promotes chromosome missegregation and damage. Finally, complete understanding of cell division and its transitions can only occur if all components and regulators of the process are identified. While the mitotic parts list is already large, continued reports of new components indicate that it is not yet complete in vertebrates. Using bioinformatic guidance that has proven highly effective, the Gorbsky lab is testing candidate mitotic regulators and characterizing their functions in cell division in normal and transformed cells.